On the basis of completed quantitative morphologic studies we have described relationships between cellular structure and cellular function. We postulate that structure in fact is a direct reflection of intercellular volume flow and can be used to gain new insight into trans-epithelial transport mechanisms. Our analytic formulation has been applied successfully to proximal convoluted and straight segments of the rabbit nephron and now will be tested for applicability and uniqueness in other transporting epithelial having different transport characteristics. For this purpose, the structure and function of renal tubules from different species and from animals before and after experimental or naturally occuring adjustments in transport rate will be studied by microperfusion and electron microscopic, morphometric techniques. In addition, using computer techniques we will test the applicability of alternate formulations also consistent with the morphologic evidence. Thus, further support for the validity of our present formulation will be gained if alternate formulations are found to be impractical or if cellular properties predicted by our theory are confirmed by proposed direct experimental measurements.